The invention relates to an MR apparatus which includes
a main field magnet for generating a uniform, steady magnetic main magnetic field in an examination zone,
a gradient coil system which includes three gradient coils for generating magnetic fields which extend parallel to the main magnetic field and have gradients extending in different directions.
The invention also relates to a gradient coil or a gradient coil system for an MR apparatus.
Such MR apparatus with a gradient coil system with three gradient coils have been known since long. The magnetic fields that can be generated by the individual gradient coils have a gradient which extends either perpendicularly or parallel to the main magnetic field. In order to generate a gradient extending in the direction of the main magnetic field use is made of gradient coils of the Helmholtz type which have two spatially separated groups of conductors which circularly enclose the examination zone (in the case of a closed magnet with a cylindrical patient opening. The gradient coils for generating gradients extending perpendicularly to the main magnetic field, however, are coils of the saddle type. In MR apparatus with closed magnets the conductors of such a gradient coil may also be present on the surface of a cylinder, but the conductors of the individual segments of this coil do not completely enclose the examination zone but cover each time only a part of the circumference of the cylindrical surface.
Because of the different construction of the gradient coils they also have different properties. For example, such differences cause a different eddy current behavior, which itself gives rise to different pulse responses. Consequently, artifacts occur in the MR image when gradients are generated which extend obliquely relative to the main magnetic field while different gradient coils are simultaneously active.
Furthermore, the energy required to generate a gradient of the same magnitude by means of the various gradient coils also differs; the gradient coils of the Helmholtz type require less energy for this purpose than gradient coils of the saddle type. As a result, the electric power required to generate a defined gradient within a given period of time is also dependent on the relevant type of coil. Therefore, the gradient amplifiers for delivering these currents for the gradient coils are also loaded to a different extent.
This fact can be taken into account by utilizing gradient amplifiers having a different maximum power, be it that the differences in the gradient channels become even larger and the described artifacts may even be intensified. Instead, however, use can also be made of identically constructed gradient amplifiers. The power reserves of the amplifier for the gradient direction coincident with the direction of the main magnetic field will not be fully utilized in that case (if the same maximum value of the gradient is to be reached in all three gradient directions and also the same slope of the gradient pulses). However, there is the advantage of a simpler configuration and there are also logistic advantages.
It is an object of the present invention to improve an MR apparatus of the kind set forth such that the differences between the various gradient coils are at least reduced. On the basis of an NMR apparatus of the kind set forth this object is achieved according to the invention in that the gradient coils are configured in such a manner that the gradients enclose angles other than 0xc2x0 and 90xc2x0 relative to the direction of the main magnetic field.
The invention is based on the recognition of the fact that the gradient coils required for generating gradients extending obliquely relative to the main magnetic field are neither pure Helmholtz coils nor pure saddle coils (that is, when they have the same, for example, cylindrical, geometrical shape). The gradient coils according to the invention instead constitute a mixture of the two types of coil. For example, in the case of a cylindrical coil shape this means that there is a conductor segment which extends (like in a Helmholtz coil) across the entire circumference (360xc2x0) of the cylinder whereas another conductor segment of the gradient coil extends over only a part of the circumference. Therefore, the properties and construction of the gradient coils for the various gradient directions do not deviate as much from one another as in an MR apparatus in which the gradients extend either in the direction of the main magnetic field or perpendicularly thereto.
The invention is used for cylindrical gradient coils as used for closed main field magnets which completely enclose the examination zone while leaving a cylindrical patient opening only at the center.
In a further feasible application which is suitable for so-called open main field magnets (or open MRI), the main field magnet has a respective magnet pole above and below the examination zone and each gradient coil consists of a pair of coils (in this context the term gradient coil is to be broadly interpreted) above and below the examination zone, the gradient coils being simultaneously traversed by a current in opposite directions. In the case of such differences the difference between the coils for generating a gradient extending in the direction of the main magnetic field and the coils for generating a gradient extending perpendicularly thereto is even much larger than in an MR apparatus with a closed magnet system.
In a preferred embodiment, three gradient coils may have the same construction and merely are rotated 12020  about the symmetry axis relative to one another. When the three coils enclose an angle of approximately 55xc2x0 relative to the direction of the main magnetic field, the directions of the three gradients extend perpendicularly to one another. However, inductive couplings may then arise between the three coils, such couplings can be avoided in the case of a smaller or a larger angle between the gradients on the one side and the direction of the main magnetic field on the other side.